1. Field of Invention
This invention relates to aluminum alloys suitable for use in aircraft applications. More specifically, it relates to a method of making an improved aluminum product having improved damage tolerant characteristics, including improved fracture toughness, fatigue resistance, corrosion resistance, formability and surface roughness properties.
2. Description of the Related Art
The design of commercial aircraft requires different sets of properties for different types of structures. Depending on the design criteria for a particular airplane component, improvements in fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety. For example, a slower fatigue crack growth rate will require a longer time for a crack or flaw to grow to a size where it becomes "critical" leading to catastrophic failure; and higher fracture toughness means that a crack can grow to a longer length before it is critical.
Corrosion damage has been a perennial problem in today's aircraft, and the fuselage is the prime location for corrosion to occur. Improvements in corrosion resistance, therefore, are often sought with or without weight savings.
The issues of toughness, fatigue and corrosion all relate to structural integrity of the airplane. Formability, on the other hand, is a manufacturing concern. Sheets that are easier to bend, stretch and draw into various shapes reduce the cost of building an airplane because the scrap rate is reduced, and less labor is required.
For some time, heat treatable aluminum base alloy sheet and plate containing copper, magnesium and manganese has found considerable acceptance for various structural members. Such alloys generally contain 3.8 to 4.9 wt. % copper, 1.2 to 1.8 wt. % magnesium and 0.3 to 0.9 wt. % manganese and carries the Aluminum Association designation of 2024 alloy. This alloy is noted for its superior strength to weight ratio, its good toughness and tear resistance, and adequate resistance to general and stress corrosion effects.
Workers in the field have generally adapted the 2024 alloy for use in the construction of commercial aircraft. For example, one alloy used on the lower wing skins of some commercial jet aircraft is alloy 2024 in the T351 temper. Alloy 2024-T351 has a relatively high strength-to-density ratio and exhibits reasonably good fracture toughness, good fatigue properties, and adequate corrosion resistance. U.S. Pat. Nos. 4,336,075 to Quist et al. and 4,294,625 to Hyatt et al. disclose an alloy which has a higher strength to density ratio, improved fatigue and fracture toughness characteristics over alloy 2024 while maintaining corrosion resistance levels approximately equal to or slightly better than 2024. Quist et al. and Hyatt et al. achieve their improvements by homogenizing the alloy at a moderate temperature, carefully controlling the hot-rolling and extrusion parameters and then natural age-hardening to produce a highly elongated, substantially unrecrystallized microstructure. Similarly, U.S. Pat. No. 5,213,639 to Colvin et al. discloses an alloy which has at least a 5.0% improvement over 2024 alloy in T-L fracture toughness or fatigue crack growth rate by re-heating the alloy prior to hot rolling.
There still remains a need, however, for an improved alloy that has increased fracture toughness, fatigue resistance, corrosion resistance and formability over alloy 2024. Accordingly, it is the primary object of this invention to provide such an alloy.